1. Field of the Invention
This invention relates to FM stereo decoding, and more particularly to stereo decoding using digital signal processing (DSP).
2. Description of the Related Art
It would be desirable to obtain the potentially distortion-free benefits offered by DSP for FM stereo decoding. As in other areas, the use of digital processing creates the potential for greater fidelity in FM audio products. Likely applications for digital processing in stereo receivers includes home audio, automotive audio and stereo television.
The standard FM stereo broadcast signal in the United States consists of an L+R (left+right) signal within the baseband frequency range of 50-15,000 Hz, an L-R signal that has been translated in frequency onto a subcarrier of 38 kHz (the subcarrier is then supressed), and a 19 kHz pilot signal that is used in the restoration of the L and R signals after demodulation of the composite signal in the receiver.
Upon reception, the received signal is demodulated to recover the composite modulating signal. The composite signal is processed through a bandpass filter that passes only frequencies in the range of 23-53 kHz, thereby extracting the L-R double-sideband signal. The 19 kHz pilot signal is also extracted with a filter, and is then passed through a frequency doubler-amplifier circuit from which a signal of 38 kHz is obtained. This signal and the output of the bandpass filter are mixed together, yielding an L-R audio signal at the 50-15,000 Hz baseband. Finally, the L+R and L-R signals are fed to a circuit which separates them into the L and R signals, in a manner inverse to that in which they were generated at the transmitter. After further amplification, the L and R signals are furnished to an appropriate stereophonic loudspeaker system.
Non-linear analog circuits are used to accomplish the frequency doubling from 19 to 38 kHz in the receiver. Unfortunately, this adds an undesirable level of distortion. In an alternate approach, a 38 kHz signal is generated at the receiver and locked to the 19 kHz pilot signal with a phase locked loop (PLL). Such systems have been implemented with both analog circuits (Krauss et al., Solid State Radio Engineering, John Wiley & Sons, 1980, pages 318-322), and with digital technology (Manlove, "A Fully Integrated High Performance FM Stereo Decoder", IEEE 1991 Custom Integrated Circuits Conf., pages 24.6.1-24.6.3). A combined analog/digital circuit with a PLL is described in Haug et al., "A DSP-Based Stereo Decoder for Automotive Radio" SAE Technical Paper Series, 1990, No. 900244. However, the use of a PLL introduces distortion, and also involves the use of a tracking loop that adds to the circuitry requirements.